1. Field of the Invention
The present invention relates generally to a manufacturing method for a multilayer ceramic substrate, and more particularly to a forming method for a surface layer improved in adhesion strength of thick film pads.
2. Description of the Related Art
Electronic apparatus such as computers and electronic exchanges for processing a large amount of information are increasingly demanded to have a small size, high reliability, and high speed. Accordingly, high-density construction of electronic circuits is demanded. This demand is conventionally met by increasing a degree of integration of a chip (semiconductor element) itself. Further, in concert with a recent demand to high-density mounting and high-speed operation, there has been developed a multichip module having a plurality of chips mounted on a multilayer ceramic substrate which has a good heat dissipation property and is suitable for high-speed operation. Such a multichip module is now used in high-speed computers, electronic exchanges, etc.
Such a multilayer ceramic substrate is manufactured by the following conventional method, for example. First, a plurality of holes are formed through a green sheet of glass-ceramics, and these holes are filled with copper paste to form a plurality of vias. Then, copper paste is screen-printed on the green sheet to form a plurality of thick film pads connected to the vias and a plurality of thick film patterns connected to the thick film pads. Then, the green sheet is dried at a temperature of about 70.degree. C. A plurality of green sheets each obtained in the above manner are laminated together and then bonded integrally with each other by applying heat and pressure. Then, the laminated green sheets thus bonded are fired to manufacture the multilayer ceramic substrate.
In many cases, surface conductor patterns are formed on a surface layer of the multilayer ceramic substrate by a thin film process, in consideration of adhesion between the conductor patterns and the substrate. However, the formation of the surface conductor patterns by the thin film process causes an increase in cost from the view-points of installation and steps. In contrast thereto, a thick film process for forming thick film pads by screen-printing a conductor paste on the surface layer have advantages of relatively inexpensive installation and simple steps, thus reducing a manufacturing cost. However, the thick film pads on the surface layer formed by the thick film process is insufficient in adhesion to the substrate. In particular, it is difficult to find a conductor paste superior in adhesion to a glass-ceramics substrate.
A conductor paste now commercially available has been developed for an alumina substrate, and a satisfactory adhesion characteristic on the alumina substrate is obtained. However, it is difficult to find a conductor paste which is superior in wettability to the glass-ceramics substrate and is enough satisfactory in adhesion to the substrate. In particular, a reduction in adhesion strength between the thick film pads on the surface layer and the glass-ceramics substrate causes a deterioration in connection strength between the thick film pads and the vias, thereby reducing reliability. This is considered to be due to the fact that the conductor paste does not match the glass-ceramics substrate.